This invention relates to an active vehicle suspension which includes an actuator capable of varying the suspension force between the vehicle sprung and unsprung masses.
A typical actuator is a double acting hydraulic actuator connected between the sprung and unsprung masses in parallel with a load supporting spring and having a pair of activating chambers with hydraulic fluid therein so that an increase in fluid pressure in one of the activating chambers relative to the other increases or decreases the vertical force supporting the sprung mass from the nominal spring force, depending on which chamber sees the excess pressure. A hydraulic fluid control apparatus is effective to control the pressure of fluid in each of the activating chambers in response to sensed vehicle dynamic parameters to produce a desired control of the vehicle sprung mass. A typical goal of such a system is to maintain the sprung mass attitude during vehicle dynamic maneuvers such as turning, braking, etc., although the system may further be designed to respond to the faster vertical motions produced by road surface disturbances to produce a desired ride quality.
In any particular system, however, some road inputs may produce an unsprung mass motion that is too quick for the normal control response time of the control. This is particularly true of sudden vertically upward impacts from bumps or objects in the road, curbs, etc. In such cases, the actuator, which may be filled with a substantially incompressible fluid, can act as a rigid member and tend to immediately transmit the shock of the disturbance directly to the sprung mass before the control has a chance to respond and adjust the suspension force. If the disturbances are quite small, they can be absorbed by a bushing between the actuator and the sprung mass. However, good suspension design places limits on the absorption capacity of such bushings; and some such disturbances exceed that capacity.
Even if the system can respond to such disturbances in time, however, it is wasteful of energy to require the actuators to produce the required response from the on-board energy supply of the hydraulic control when the energy needed for response to a vertically upward road disturbance is potentially provided by the road itself. All that is really necessary is the unloading of the actuator by opening the activating chambers to each other so as to allow the road disturbance to collapse the actuator as necessary without communicating the disturbance through the actuator to the sprung mass. However, the timely sensing of the road input and very fast response required may be beyond the capabilities of the control, particularly if it is microprocessor based.